Hitherto, an olefinic polymer-based viscosity index improver has been widely employed in engine oil due to its excellent thickening effect. The viscosity index improver of this type is distributed as a commercial product in the form of a solution comprising approximately 10 to 20% of an olefinic polymer and approximately 90 to 80% of a mineral oil. If the proportion of the olefinic polymer component is increased above this range, the viscosity of the product is too large, causing handling problems. As such, the product is not of practical use. Accordingly, a viscosity index improver product which contains an olefinic polymer in a high concentration but which has a low viscosity is needed. In order to satisfy this need, some methods have been proposed. For instance, Japanese Patent Kokai No. 171417/83 discloses a method of incorporating a graft or block copolymer of an olefinic copolymer and a (meth)acrylate into a solvent which does not substantially dissolve or which scarcely dissolves the olefinic copolymer, for use as a dispensing agent to obtain a high-concentration product. Japanese Patent Kokai No. 171418/83 discloses another method of incorporating a graft or block copolymer comprising an olefinic copolymer and vinyl monomer(s) other than (meth)acrylates into a solvent which does not substantially dissolve or which scarcely dissolves the olefinic copolymer, for use as a dispersing agent, also to obtain a high-concentration product. However, these methods of preparing the dispersing agent are troublesome since preparation of the dispersing agent to be used requires more (meth)acrylates than theoretically needed and the dispersing agent must always be used in a large amount (for example, 12% or more), except when a styrene-isoprene block copolymer is used as a dispersing agent. Such dispersing agents are, not efficient. In addition, when a styrene-isoprene copolymer is used as a dispersing agent, the product causes haze or cloudiness when dissolved in mineral oil. Such haze or cloudiness often causes problems in practical use.
Regarding methods of preparing the products, the above-mentioned Japanese Patent Kokai Nos. 171417/83 and 171418/83 discloses a method of adding a granular olefinic copolymer into a dispersing agent-containing poor solvent and emulsifying them with a particular high shearing force stirrer. However, the method does not use a standard or conventional stirrer, but rather a particular stirrer. Additionally, since a granular olefinic copolymer is required, the propylene content in the olefinic copolymer is inevitably to be approximately 30% or less or approximately 70 or more. (This is because if the propylene content is 40 to 60% the olefinic copolymer itself would be soft and therefore could hardly be shaped into granules ) Further, where the olefinic polymer is degraded or modified in a solvent solution prior to emulsification, the resulting solvent solution of the degraded or modified olefinic polymer could not be directly used. As such, the disclosed method has various unfavorable problems.
The present inventors have studied and investigated various efficient methods of high-concentration non-aqueous dispersion of low-viscosity olefinic polymers and, as a result, have achieved the present invention.